Thursday, March 13, 2008

A. Review how clouds are formed, that they are made of condensed water vapor.

B. Precipitation - about 10% of clouds produce precipitation, which is defined as any form of water falling from the atmosphere. Most of the precipitation is in the form of rain and drizzle. Inside a cloud, a water droplet is about 4/10,000th of an inch in diameter!! Tiny!!!! But these microscopic droplets of water are moving up and down inside the cloud, and as they fall, they bunch together or coalesce and become bigger. (Think of how raindrops run together into bigger drops as they run down a window) When a raindrop reaches 2/100th of an inch in diameter it is considered drizzle. Raindrops earn the 'official' designation of "rain" when they are bigger than that, up to 1/4 inch.

C. When it rains and the temperature near the ground is below freezing, we say that we're getting "freezing rain" or an ice storm, because the rain freezes as it hits the ground.

D. We made a simple rain gauge using baby food jars. I don't know how accurate they will be, but it will be interesting to compare results.

E. Precipitation may also be in the form of a solid - snow, sleet, or hail. When rain falls through very cold air, it can start to freeze into small ice pellets that we call sleet. Hailstones are larger ice pellets, frozen in layers as they move up and down inside the cloud, before falling as hail. Snow occurs when water vapor crystallizes in the clouds and doesn't melt before falling to earth. Snowflakes are always six-sided and no two are identical. Dew and frost both are formed when water vapor condenses on the ground. This usually happens overnight.